In a typical cellular radio system, wireless terminals communicate via a Radio Access Network (RAN) to one or more Core Networks (CN). The wireless terminals are also known as mobile stations and/or user equipment units, such as mobile telephones, smart phones, cellular telephones, tablet computers and laptops with wireless capability, e.g., mobile termination, and thus may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices which communicate voice and/or data via the radio access network. In the following, the term user equipment is used when referring to the wireless terminal.
The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a Base Station (BS), e.g., a Radio Base Station (RBS), which in some networks is also called NodeB, B node, evolved Node B (eNB) or Base Transceiver Station (BTS). The term base station will be used in the following when referring to any of the above examples. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. The base stations communicate over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions, particularly earlier versions, of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the base station(s) connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. Universal Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units. The Third Generation Partnership Project (3GPP) has undertaken to further evolve the UTRAN and GSM based radio access network technologies.
Long Term Evolution (LTE) is a variant of a 3GPP radio access technology wherein the radio base station nodes are connected directly to a core network, rather than to radio network controller nodes. In LTE the functions of a radio network controller node are generally performed by the radio base station nodes. As such, the radio access network of an LTE system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller nodes. LTE was introduced in 3GPP with release 8. Release 9 and release 10 are later releases of LTE. For example, release 8 may be referred to as e.g. rel-8, release 8, LTE release 8 or 3GPP release 8. The terms “codeword,” “layer,” “precoding,” and “beam forming” have been adapted specifically for LTE to refer to signals and their processing. A codeword represents user data before it is formatted for transmission. The term “layer” is synonymous with stream. For Multiple Input Multiple Output (MIMO), at least two layers must be used. Up to four are allowed. The number of layers is always less than or equal to the number of antennas. Precoding modifies the layer signals before transmission. This may be done for diversity, beam steering, or spatial multiplexing. Beam forming modifies the transmit signals to give the best Carrier to Interference-plus-Noise Ratio (CINR) at the output of the channel.
In LTE, Hybrid Automatic Repeat reQuest (HARQ) with incremental redundancy is used. HARQ is a technique that enables faster recovery from errors in communication networks by storing corrupted packets in the receiving device rather than discarding them. Even if retransmitted packets have errors, a good packet may be derived from the combination of bad ones. Instead of re-transmitting the same portion of the codeword, different redundancy versions are re-transmitted yielding an extra gain over Chase combining.
Ideally, a full buffer is available at the receiver side such that the received soft values for an entire codeword may be stored. However, due to the user equipment complexity and cost concerns, the soft buffer size in a user equipment is limited. For higher rate transmissions, where larger codewords are sent from the transmitter, the user equipment has limited buffer space and is not able to store the complete codeword. The base station may transmit coded bits the user equipment is not able to store, or worse, the user equipment does not know that these are other bits and confuses them with bits it previously has stored.
FIG. 1 depicts simplified a complete codeword and also how many soft bits the user equipment is able to store. FIG. 1 illustrates an encoded transport block and coded bits stored by the user equipment, i.e. soft buffer size. As seen in FIG. 1, the complete codeword comprises systematic bits and parity bits, and the soft buffer size comprises all systematic bits and some of the parity bits of the complete codeword. A parity bit is a bit that is added to a group of source bits to ensure that the number of set bits in the outcome is even or odd. The parity bit may be used to detect single or any other odd number of errors in an output. If the base station and the user equipment have the same understanding about the soft buffer size, then the base station will not transmit coded bits which the user equipment is not able to store. Instead, it only takes those coded bits stored by the user equipment and uses those bits for (re)transmissions. This is depicted by the circular buffer shown in FIG. 2. The term circular buffer refers to an area in a memory which is used to store incoming data. When the buffer is filled, new data is written starting at the beginning of the buffer and overwriting the old data. The codeword, i.e. the systematic bits and the parity bits, are stored in the circular buffer. FIG. 2 illustrates bits used in a first transmission and re-transmissions, derived from the circular buffer. The size of the circular buffer matches the soft buffer size of the user equipment. The complete circle in FIG. 2 corresponds to the soft buffer size and not to the entire codeword. In the first transmission, depending on the code rate, some or all systematic bits, and none or some parity bits are transmitted. In a retransmission the starting position is changed and bits corresponding to another part of the circumference, e.g. another point in the circular buffer, are transmitted.
In LTE release 8 using Frequency-Division Duplexing (FDD), each user equipment has up to 8 HARQ processes per component carrier. Each HARQ process may comprise up to two sub-processes for supporting dual-codeword MIMO transmissions. Release 8 of LTE divides the available soft buffer equally into the configured number of HARQ processes. Each of the divided soft buffers may be used to store soft values of the received codewords. In case of dual-codeword MIMO transmission, the divided soft buffer is further divided equally to store the soft values of the two received codewords.
In 3GPP the soft buffer size allocation is provisioned as below:
The circular buffer wk for the r-th coded block is generated as follows:wk=vk(0) for k=0, . . . ,KΠ−1wkΠ+2k=vk(1) for k=0, . . . ,KΠ−1wkΠ+2k+1=vk(2) for k=0, . . . ,KΠ−1where KΠ is a constant.
The circular buffer has the length Kw=3KΠ.
Denote the soft buffer size for the transport block by NIR bits and the soft buffer size for the r-th code block by Ncb bits. The size Ncb is obtained as follows, where C is the number of code blocks:
      N    cb    =      min    ⁡          (                        ⌊                                    N              IR                        C                    ⌋                ,                  K          w                    )      for downlink turbo coded transport channelsNcb=Kw for uplink turbo coded transport channels,where NIR is equal to:
      N    IR    =      ⌊                  N        soft                              K          MIMO                ·                  min          ⁡                      (                                          M                DL_HARQ                            ,                              M                limit                                      )                                ⌋  where:Nsoft is the total number of soft channel bit.KMIMO is equal to 2 if the user equipment is configured to receive Physical Downlink Shared Channel (PDSCH) transmissions based on transmission modes 3, 4 or 8, 1 otherwise.MDL_HARQ is the maximum number of DL HARQ processes.Mlimit is a constant equal to 8.
The Soft Buffer (SB) allocation for the single-codeword transmission modes is illustrated in FIG. 3. FIG. 3 illustrates 8 allocated soft buffers, where SB0 illustrates a first soft buffer for a first codeword, SB1 illustrates a second soft buffer for a second codeword, SB2 illustrates a third soft buffer for a third codeword etc. FIG. 3 shows soft buffer allocation in LTE release 8 when the Physical Downlink Shared Channel (PDSCH) transmission mode is other than mode 3, 4 or 8. It may be observed that there is a buffer reserved for each codeword.
The soft buffer allocation for the dual-codeword transmission modes is illustrated in FIG. 4. FIG. 4 illustrates 16 allocated soft buffers, where SB0a illustrates a first buffer for a first codeword, SB0b illustrates a second buffer for a second codeword, SB1 a illustrates a third buffer for a third codeword, SB1 b illustrates a fourth soft buffer for a fourth codeword etc. The soft buffer applies to a codeword. The codeword is a term used for the coded bits associated with a transport block. FIG. 4 shows soft buffer allocation in release 8 of LTE when the PDSCH transmission mode is mode 3, 4 or 8. The transmission modes will be described in more detail below.
The buffer reserved for each codeword is only half of the previous operating case. The soft buffer limitation problem is particularly acute in dual-codeword MIMO transmission operations. This limitation reduces the effectiveness of soft combining gains from incremental redundancy retransmissions.
Carrier Aggregation.
The release 8 of LTE supports bandwidths up to 20 Mega Hertz (MHz). However, in order to meet the International Mobile Telecommunications-Advanced (IMT-Advanced) requirements, 3GPP initiated work on LTE release 10. One part of LTE release 10 is to support bandwidths larger than 20 MHz. An important requirement for LTE release 10 is to assure backward compatibility with LIE release 8, including spectrum compatibility. As a result, a carrier of LTE release 10, which is wider than 20 MHz, may appear as a number of smaller LIE carriers to a user equipment of LIE release 8. Each such carrier may be referred to as a component carrier or cells. For early LTE release 10 deployments, it may be expected that there will be a smaller number of LIE release 10-capable user equipments compared to many LTE legacy user equipments. Therefore, it is desirable to assure an efficient use of a wide carrier by legacy user equipments, which means that it may be possible to implement carriers where legacy user equipments may be scheduled in all parts of the wideband LTE release 10 carrier. One way to achieve this would be using Carrier Aggregation (CA).
Carrier aggregation implies that a user equipment supporting LTE release 10 may receive multiple component carriers, where the component carriers have, or at least may have, the same structure as a carrier of LIE release 8. Carrier Aggregation is illustrated in FIG. 5. The x-axis of FIG. 5 denotes the width of the spectrum used for the five component carriers and the y-axis defines the energy per frequency unit.
Soft Buffer Operation in Carrier Aggregation.
In LTE each component carrier operates with its own set of HARQ processes. Since the total soft buffer memory needs to be shared among component carriers, the soft buffer size per component carrier may vary depending on the number of configured component carriers and the number of configured MIMO transmission modes for each component carriers. The available soft buffer size for each codeword also depends on how the soft buffer is divided and allocated amongst all codewords.
Multi-Antenna Support in LTE.
Multi-antenna capabilities are included already in release 8 of LTE, and are important enablers for high data rates, improved coverage and capacity. The multiple antennas at transmitters and receivers may be used in different ways. Diversity techniques are used to improve the robustness of the link. Beam-forming techniques may be used to improve the coverage. Spatial multiplexing provides a means to enhance the spectral efficiency of the link and improves the performance of the whole system if properly designed. Peak rates may be substantially increased using spatial multiplexing and is ideally be increased proportionally to the minimum number of transmit and receive antennas of the link, provided that the Signal-to-Noise Ratio (SNR) is high enough and that the channel conditions are beneficial. Realistic gains are highly channel dependent, they require a high SNR and beneficial interference situations of the relevant link, but may be substantially improved provided that the SNR is sufficiently high. Examples are low system load scenarios or when the user equipment is close to the cell center.
The downlink in LTE release 8 supports Single-User MIMO (SU-MIMO) spatial multiplexing of up to four layers via codebook based precoding. In addition, transmit diversity modes as well as beamforming with single-layer transmission are supported in the downlink of LTE release 8. In LTE release 9, an enhanced downlink transmission mode is introduced in which the beamforming functionality is extended to also support dual-layer transmission, and in which Multi-User MIMO (MU-MIMO) operation is offered where different layers are transmitted to different users. The uplink multi-antenna support in LTE release 8/9 is limited to user equipment antenna selection, which is optional in all UE categories. The UE categories will be described in more detail below.
A user equipment of LTE release 8 assumes its number of layers based on the minimum of what the base station supports and what the user equipment supports. The user equipment determines how many layers the base station supports by either blindly detecting how many Cell-specific Reference Signal (CRS) antenna ports the base station is transmitting from, or in the case of a HandOver (HO), by receiving the information about the how many antenna ports the target cells supports in the HO-command.
Multi-antenna transmission is an important feature in LTE release 8. LTE supports the following 8 transmission modes (TM):                Mode 1: Single antenna port.        Mode 2: Transmit diversity.        Mode 3: Open-loop spatial multiplexing.        Mode 4: Closed-loop spatial multiplexing.        Mode 5: MU-MIMO.        Mode 6: Closed-loop spatial multiplexing, single layer.        Mode 7: Single antenna port, user equipment specific reference signal.        Mode 8: Single or dual-layer transmission with user equipment specific reference signal.        
LTE-Advanced, i.e. LTE release 10, comprises a mode 9, in addition to modes 1-8. Mode 9 is a multilayer transmission mode supporting closed-loop SU-MIMO up to rank 8 and enhanced MU-MIMO support.
UE Category Signaling.
User equipments may be categorized in different user equipment categories, called UE categories or UE classes, which defines the overall performance and capabilities of the user equipment. The user equipment category is from now on referred to as UE category. The UE categories are needed to ensure that the base station may communicate correctly with the user equipment. By letting the base station know the UE category, it is able to determine the performance of the user equipment and communicate with it accordingly.
As the UE category defines the overall performance and the capabilities of the user equipment, it is possible for the base station to communicate using capabilities that it knows the user equipment possesses. Accordingly, the base station will not communicate beyond the performance of the user equipment. Different values of a buffer size are associated with each UE category.
In LTE release 8/9, there are five UE categories, 1-5. LTE release 10 has three additional categories, 6-8.
The definition of UE categories of LTE release 10 builds upon the principles used in LTE release 8/9, where the number of UE categories is limited to avoid fragmentation of user equipment implementation variants in the market. The LTE release 10 UE categories are defined in terms of peak rate, ranging from 10, 50, 100, 150 and 300 Mbps up to about 3 Gbps in the downlink. Different realizations of the peak rates are possible within a UE category. For example, in categories 6 and 7, it is possible to either support two layers of MIMO together with carrier aggregation of 40 MHz, or four layers of MIMO with a single carrier of 20 MHz. Both configurations support up to 300 Mbps. The LTE release 8/9 UE categories are reused, supporting, e.g. aggregation of two component carriers with up to 10 MHz bandwidth each for a user equipment of category 3. It is expected that additional UE categories may be defined in the future. LTE release 10 supports a high-end UE category combining aggregation of five component carriers of 20 MHz each with eight layer MIMO, which supports a total peak data rate of about 3 Gbps for LTE-Advanced. Table 1 below shows UE categories supported in LTE release 10. The left most column comprises the UE categories 1-8. The next column comprises the maximum number of DownLink-Shared CHannel (DL-SCH) transport block bits received within a Transmission Time Interval (TTI). The middle column comprises the maximum number of bits of a DL-SCH transport block received within a TTI. The column to the right of the middle column comprises the total number of soft channel bits. The right most column comprises the maximum number of supported layers for spatial multiplexing in DL. Spatial multiplexing is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data signals from each of the multiple transmit antennas.
TABLE 1UE categories supported in LTE release 10MaximumMaximumnumber ofnumber ofDL-SCHMaximum numberTotalsupportedtransportof bits of a DL-SCHnumber layers forblock bitstransport blockof softspatialUE receivedreceived within channelmultiplex-Categorywithin a TTIa TTIbitsing in DL110296102962503681251024510241237248231020487537612372482415075275376182707225299552149776 366720046301504149776 (4 layers)36672002 or 4 75376 (2 layers)7301504149776 (4 layers)36672002 or 4 75376 (2 layers)82998560299856 359827208
The user equipment capability signaling of UE categories is defined in the following way. LTE release 8/9 categories 1-5 are signaled from the user equipment to the base station via the Radio Resource Control (RRC) protocol. The RRC protocol handles the control plane signaling of Layer 3 between the user equipment and the UTRAN. LTE release 10 categories are signaled from the user equipment to the base station in separately via the RRC protocol, using a LTE release 10 part of the RRC protocol. The receiver of the message is the base station, and it also the base station that uses the received information. However, the user equipment is not aware of the release of the base station. So in order to be able to operate in a legacy network, a LTE release 10 user equipment would thus report both a LTE release 8/9 UE category (1-5) using a LTE release 8/9 part of the RRC protocol and a LTE release 10 UE category (6-8) using a LTE release 10 part of the RRC protocol. The LTE release 10 UE category would be understood by a LTE release 10 base station, but not by the LTE release 8/9 base station. In addition, a LTE release 10 user equipment also informs the base station per frequency band combination about the supported number of supported MIMO layers in UpLink (UL) and DownLink (DL), as well as the number of supported aggregated component carriers. This information is only understood by a LTE release 10 base station.
As an example, a LTE release 10 user equipment, e.g., category 6, indicates to a LTE release 10 base station that it supports up 4 MIMO layers in the DownLink (DL). The LTE release 10 user equipment may provide this MIMO layer information in an Information Element (IE) sent in addition to the category values. This information element is understood by a LTE release 10 base station but ignored by a LTE release 8/9 base station. A LTE release 8 base station that supports 4 MIMO layers in DL identifies the user equipment through its LTE release 8/9 category e.g., category 4, and therefore assumes that the user equipment supports only 2 layers of DL MIMO.
Because the user equipment is not aware of the release of the base station, it does not know whether to operate according to an older release, e.g., the LTE release 8/9 category, e.g. category 4, or a newer release, e.g., the LTE release 10 category, e.g. category 6. This has serious consequences as the user equipment operates differently depending on the category. In this example, the user equipment may assume that the base station operates according to 4 layer MIMO in DL, as it detects the Cell Reference Signal (CRS) pattern according to 4 layer MIMO, and sends feedback to the base station to support 4 layer DL MIMO operation such as a rank indicator greater than 2, Channel Quality Indicator (CQI), and Pre-coding Matrix Index (PMI). But this leads to corrupted UpLink (UL) control signaling as the base station assumes a maximum rank of 2 when decoding the control signaling, according to the LTE release 8/9 category, e.g. category 4, indicated by the user equipment. It may also lead to corrupted UL data if the UL data is multiplexed together with the UL control signaling.
As another example, a LTE release 10 user equipment may support a higher number of DL MIMO layers than what is required by the UE category. If the user equipment operates according to the higher number of DL MIMO layers in a base station that does not operator with this higher number of DL MIMO layers, similar problems arise as described above.